--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/clang-tools-extra/clangd/support/Context.h	Mon May 25 11:55:54 2020 +0900
@@ -0,0 +1,222 @@
+//===--- Context.h - Mechanism for passing implicit data --------*- C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Context for storing and retrieving implicit data. Useful for passing implicit
+// parameters on a per-request basis.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_CONTEXT_H_
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_CONTEXT_H_
+
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/Compiler.h"
+#include <memory>
+#include <type_traits>
+
+namespace clang {
+namespace clangd {
+
+/// Values in a Context are indexed by typed keys.
+/// Key<T> serves two purposes:
+///   - it provides a lookup key for the context (each Key is unique),
+///   - it makes lookup type-safe: a Key<T> can only map to a T (or nothing).
+///
+/// Example:
+///    Key<int> RequestID;
+///    Key<int> Version;
+///
+///    Context Ctx = Context::empty().derive(RequestID, 10).derive(Version, 3);
+///    assert(*Ctx.get(RequestID) == 10);
+///    assert(*Ctx.get(Version) == 3);
+///
+/// Keys are typically used across multiple functions, so most of the time you
+/// would want to make them static class members or global variables.
+template <class Type> class Key {
+public:
+  static_assert(!std::is_reference<Type>::value,
+                "Reference arguments to Key<> are not allowed");
+
+  constexpr Key() = default;
+
+  Key(Key const &) = delete;
+  Key &operator=(Key const &) = delete;
+  Key(Key &&) = delete;
+  Key &operator=(Key &&) = delete;
+};
+
+/// A context is an immutable container for per-request data that must be
+/// propagated through layers that don't care about it. An example is a request
+/// ID that we may want to use when logging.
+///
+/// Conceptually, a context is a heterogeneous map<Key<T>, T>. Each key has
+/// an associated value type, which allows the map to be typesafe.
+///
+/// There is an "ambient" context for each thread, Context::current().
+/// Most functions should read from this, and use WithContextValue or
+/// WithContext to extend or replace the context within a block scope.
+/// Only code dealing with threads and extension points should need to use
+/// other Context objects.
+///
+/// You can't add data to an existing context, instead you create a new
+/// immutable context derived from it with extra data added. When you retrieve
+/// data, the context will walk up the parent chain until the key is found.
+class Context {
+public:
+  /// Returns an empty root context that contains no data.
+  static Context empty();
+  /// Returns the context for the current thread, creating it if needed.
+  static const Context &current();
+  // Sets the current() context to Replacement, and returns the old context.
+  // Prefer to use WithContext or WithContextValue to do this safely.
+  static Context swapCurrent(Context Replacement);
+
+private:
+  struct Data;
+  Context(std::shared_ptr<const Data> DataPtr);
+
+public:
+  /// Same as Context::empty(), please use Context::empty() instead.
+  /// Constructor is defined to workaround a bug in MSVC's version of STL.
+  /// (arguments of std::future<> must be default-constructible in MSVC).
+  Context() = default;
+
+  /// Copy operations for this class are deleted, use an explicit clone() method
+  /// when you need a copy of the context instead.
+  Context(Context const &) = delete;
+  Context &operator=(const Context &) = delete;
+
+  Context(Context &&) = default;
+  Context &operator=(Context &&) = default;
+
+  /// Get data stored for a typed \p Key. If values are not found
+  /// \returns Pointer to the data associated with \p Key. If no data is
+  /// specified for \p Key, return null.
+  template <class Type> const Type *get(const Key<Type> &Key) const {
+    for (const Data *DataPtr = this->DataPtr.get(); DataPtr != nullptr;
+         DataPtr = DataPtr->Parent.get()) {
+      if (DataPtr->KeyPtr == &Key)
+        return static_cast<const Type *>(DataPtr->Value->getValuePtr());
+    }
+    return nullptr;
+  }
+
+  /// A helper to get a reference to a \p Key that must exist in the map.
+  /// Must not be called for keys that are not in the map.
+  template <class Type> const Type &getExisting(const Key<Type> &Key) const {
+    auto Val = get(Key);
+    assert(Val && "Key does not exist");
+    return *Val;
+  }
+
+  /// Derives a child context
+  /// It is safe to move or destroy a parent context after calling derive().
+  /// The child will keep its parent alive, and its data remains accessible.
+  template <class Type>
+  Context derive(const Key<Type> &Key,
+                 typename std::decay<Type>::type Value) const & {
+    return Context(std::make_shared<Data>(
+        Data{/*Parent=*/DataPtr, &Key,
+             std::make_unique<TypedAnyStorage<typename std::decay<Type>::type>>(
+                 std::move(Value))}));
+  }
+
+  template <class Type>
+  Context
+  derive(const Key<Type> &Key,
+         typename std::decay<Type>::type Value) && /* takes ownership */ {
+    return Context(std::make_shared<Data>(
+        Data{/*Parent=*/std::move(DataPtr), &Key,
+             std::make_unique<TypedAnyStorage<typename std::decay<Type>::type>>(
+                 std::move(Value))}));
+  }
+
+  /// Derives a child context, using an anonymous key.
+  /// Intended for objects stored only for their destructor's side-effect.
+  template <class Type> Context derive(Type &&Value) const & {
+    static Key<typename std::decay<Type>::type> Private;
+    return derive(Private, std::forward<Type>(Value));
+  }
+
+  template <class Type> Context derive(Type &&Value) && {
+    static Key<typename std::decay<Type>::type> Private;
+    return std::move(*this).derive(Private, std::forward<Type>(Value));
+  }
+
+  /// Clone this context object.
+  Context clone() const;
+
+private:
+  class AnyStorage {
+  public:
+    virtual ~AnyStorage() = default;
+    virtual void *getValuePtr() = 0;
+  };
+
+  template <class T> class TypedAnyStorage : public Context::AnyStorage {
+    static_assert(std::is_same<typename std::decay<T>::type, T>::value,
+                  "Argument to TypedAnyStorage must be decayed");
+
+  public:
+    TypedAnyStorage(T &&Value) : Value(std::move(Value)) {}
+
+    void *getValuePtr() override { return &Value; }
+
+  private:
+    T Value;
+  };
+
+  struct Data {
+    // We need to make sure Parent outlives the Value, so the order of members
+    // is important. We do that to allow classes stored in Context's child
+    // layers to store references to the data in the parent layers.
+    std::shared_ptr<const Data> Parent;
+    const void *KeyPtr;
+    std::unique_ptr<AnyStorage> Value;
+  };
+
+  std::shared_ptr<const Data> DataPtr;
+};
+
+/// WithContext replaces Context::current() with a provided scope.
+/// When the WithContext is destroyed, the original scope is restored.
+/// For extending the current context with new value, prefer WithContextValue.
+class LLVM_NODISCARD WithContext {
+public:
+  WithContext(Context C) : Restore(Context::swapCurrent(std::move(C))) {}
+  ~WithContext() { Context::swapCurrent(std::move(Restore)); }
+  WithContext(const WithContext &) = delete;
+  WithContext &operator=(const WithContext &) = delete;
+  WithContext(WithContext &&) = delete;
+  WithContext &operator=(WithContext &&) = delete;
+
+private:
+  Context Restore;
+};
+
+/// WithContextValue extends Context::current() with a single value.
+/// When the WithContextValue is destroyed, the original scope is restored.
+class LLVM_NODISCARD WithContextValue {
+public:
+  template <typename T>
+  WithContextValue(const Key<T> &K, typename std::decay<T>::type V)
+      : Restore(Context::current().derive(K, std::move(V))) {}
+
+  // Anonymous values can be used for the destructor side-effect.
+  template <typename T>
+  WithContextValue(T &&V)
+      : Restore(Context::current().derive(std::forward<T>(V))) {}
+
+private:
+  WithContext Restore;
+};
+
+} // namespace clangd
+} // namespace clang
+
+#endif